1. Field of the Invention
The present invention relates to a toner for developing latent electrostatic images. In particular, it is concerned with a negatively chargeable toner used in the field of electrophotography and electrostatic printing.
2. Discussion of Background
For developing latent electrostatic images to visible images there are two types of a developer, as conventionally disclosed in Japanese Laid-Open Patent Application 61-147261. One is a two-component type developer prepared by mixing toner and carrier particles, and the other is a one-component type developer which is obtained by dispersing a coloring agent such as a dye and a pigment in a binder resin without using carrier particles.
In the aforementioned two-component type developer, toner particles and carrier particles are electrically charged to different polarities in the course of stirring in a development unit, and the toner particles are attracted to latent electrostatic images which are charged to an opposite polarity to that of the toner particles and visible toner images can be thus obtained. The method of developing latent electrostatic images to visible toner images varies depending on the types of the employed carrier particles. For example, in the magnetic brush development, toner particles are mixed together with iron powders serving as carrier particles, and in the cascade development, toner particles are attracted to the surfaces of glass beads serving as carrier particles by the aid of a triboelectric effect. In addition, fur brush development can be carried out by using a fur brush instead of carrier particles.
In the case of a one-component type developer, various development methods are also proposed. For example, powder cloud development, one of the aerosol development methods, is conducted by spraying the one-component type developer to a transfer sheet bearing latent electrostatic images and contact development, also referred to as touch down development, is conducted in such a manner that toner particles are directly brought into contact with latent electrostatic images. There is also proposed an induction development, in which magnetic electroconductive toner particles are brought into contact with latent electrostatic images.
The toner particles applicable for the above-mentioned development methods are prepared by dispersing a coloring agent such as carbon black in a binder resin such as natural resins and synthetic resins. More specifically, a coloring agent is dispersed in a binder resin such as polystyrene, the resulting dispersion is cooled to room temperature to prepare a solid material, and then it is pulverized until a particle diameter thereof will attain to 1 to 30 .mu.m. Furthermore, by adding a magnetic material such as magnetite to the above-mentioned coloring agent and binder resin, a magnetic toner can be obtained.
As previously mentioned, toner particles applicable for various development methods are electrically charged to a positive or negative polarity depending on the polarity of the latent electrostatic image. Toner particles can be provided with the electric charge by means of the triboelectric characteristics of a resin component contained therein. In such a case, however, the chargeability of toner particles is not so sufficient that the fogging will easily occur on the obtained images, which deteriorates the sharpness of images.
To give the desired chargeability to toner particles, a dye or pigment capable of providing toner particles with electric charge, or a charge controlling agent is generally added to toner particles.
For charge controlling agents which apply a negative charge to the toner, metal complex salts of monoazo dye; nitrofumic acid and salts thereof; Co, Cr and Fe metal complexes containing salicylic acid, naphthoic acid or dicarboxylic acid; sulfonated copper phthalocyanine pigment; a nitro group- or halogen-introduced styrene oligomer; chlorinated paraffin; and melamine resin can be employed.
However, the above-mentioned dyes capable of applying a negative charge to the toner have the shortcomings that their structures are complicated and characteristics are unstable, so that the stability cannot be ensured as the charge controlling agent. In addition to this, they are easily decomposed while kneaded with application of heat thereto, and decomposed or deteriorated by mechanical shocks, frictions, and changes in temperature and humidity conditions. This will be accompanied by the deterioration of charge controllability. Some of the above dyes serving as charge controlling agent may change their charge controlling performance in accordance with the environment.
Furthermore, when toner particles containing the conventional charge controlling agents are practically used for a long period of time, the charge controlling agents are separated from the toner particles due to the friction between toner particles and between the toner particles and the surface of a photoconductor, and the collision of the toner particles with carrier particles. The charge controlling agents which have been separated from the toner are deposited to the surface of the photoconductor, with the result that a so-called toner-filming phenomenon takes place.